Zhifei Wen MS, Presenter: Nothing to Disclose

Norbert Joseph Pelc ScD, Abstract Co-Author: Nothing to Disclose

Walter R Nelson PhD, Abstract Co-Author: Nothing to Disclose

Rebecca Fahrig PhD, Abstract Co-Author: Nothing to Disclose

Previous work has shown that when an x-ray tube is placed inside a magnetic field (B), the x-ray exposure increases with increasing B. It was hypothesized that the increase was caused by back-scattered electrons which were constrained by B and re-accelerated by the electric field (E) onto the x-ray tube target. We performed computer simulations and physical experiments to study the behavior of the back-scattered electrons in a magnetic field, and their effects on radiation output and off-focal radiation.

A Monte-Carlo program (EGS4) was used to generate the combined energy and angular distribution of the back-scattered electrons. The electron trajectories were traced and their landing locations back on the anode were calculated. Radiation emission from each point was modeled by interpolation of published data. The point spread function for a pencil beam of electrons was generated and then convolved with the density map of primary electrons incident on the anode as simulated with a finite element program (Opera-3d, Vector Fields, UK). Therefore, the spatial distribution of x-ray emission and total exposure contributed by the back-scattered electrons could be predicted. Experimental measurements used an x-ray tube (10º anode), a 30μm pinhole, a flat panel x-ray detector, a calibrated dosimeter, and a 0.5T magnet (Signa-SP, GE Healthcare, Milwaukee, WI). Measurements of exposure rate and focal spot images were acquired as functions of B. Alignment between the cathode-anode axis and B was maintained for all experiments.

Simulations showed that at 65kVp ~54% of the electrons incident on the target were backscattered. Calculations at 0.5T showed that the exposure would be increased by 31.5%, and ~39% of the increase was within an area twice the size of the focal spot. Good agreement was observed between simulated results and experimental data in both total exposure and intensity distribution at various kVp’s.

Computer simulations strongly support the hypothesis that the increased emission from an x-ray tube placed within a strong B is caused by back-scattered electrons. A majority of the increased radiation is off-focal when B<0.5T.

N.J.P.: Research support, GE healthcare

Wen, Z, Pelc, N, Nelson, W, Fahrig, R, Investigation of Increase in Radiation When an X-ray Tube Is Placed in a Magnetic Field.  Radiological Society of North America 2005 Scientific Assembly and Annual Meeting, November 27 - December 2, 2005 ,Chicago IL. http://archive.rsna.org/2005/4415416.html